The present invention relates to valve control systems for internal combustion engine poppet valves, and more particularly, to such valve control systems which are capable of controlling the amount of the valve lift, the timing of the valve lift, and the duration of the valve event (the valve lift).
As is well known to those skilled in the internal combustion engine art, conventional camshaft and rocker arm type valve gear trains are relatively simple and have been generally effective in commercial use. However, the conventional camshaft-actuated valve gear train has typically represented a compromise in regard to engine performance. At relatively low speeds and loads, the engine poppet valves open more than is needed, while at relatively higher engine speeds, the valves do not open enough to get the flow quantity of air-fuel mixture necessary to achieve optimum engine performance. At relatively low speeds, if the amount of valve opening could be reduced, such that the poppet valve could serve as a flow xe2x80x9cthrottlexe2x80x9d, the engine pumping losses could be reduced.
In addition, it is now understood that engine efficiency can be improved by varying the timing of the opening and closing of the poppet valves as a function of engine speed, and also as a function of load on the engine. One known method of varying the timing of the opening and closing of the engine poppet valves is by means of a variable cam phase change device (xe2x80x9cvariable cam phaserxe2x80x9d). The function of such a variable cam phaser device is to vary the angular position of the camshaft, relative to the angular position of the crankshaft. However, providing the typical internal combustion engine with variable cam phaser capability would add substantially to the overall cost of the engine.
Those skilled in the valve gear train art have, for many years, been developing various systems for variable valve actuation/variable valve timing (xe2x80x9cVVA/VVTxe2x80x9d) for modifying the amount of valve lift and/or the timing of the valve lift in valve gear trains of the type driven by a camshaft. Those developments may be divided into several categories in order to better understand design approaches followed by the prior art, and also to better understand the design philosophy and benefits of the present invention.
In a first category are those VVA/VVT mechanisms which are able to achieve xe2x80x9cliftxe2x80x9d of the engine poppet valve in response to oscillation of a cam member, wherein, the movement of the cam member in a first direction occurs in response to rotation of the camshaft, but the xe2x80x9creturnxe2x80x9d movement in the second, opposite direction, permitting the poppet valve to close, requires a biasing spring. An example of such a mechanism is illustrated in U.S. Pat. No. 6,019,076.
As is well known to those skilled in the art, there are a number of disadvantages to such a mechanism which requires a biasing spring. First, if the oscillating cam is moved in the second direction by means of a biasing spring, then each time the oscillating cam moves in the first direction, the biasing force of the spring must be overcome, thus substantially increasing the overall energy consumption by the mechanism. In addition, springs of the type required for such a mechanism tend to be large and expensive, thus substantially increasing the overall size, weight, and cost of the mechanism. Also, it is fairly common for springs to exhibit a variable spring force over the life of the spring, thus introducing an undesirable variability, over time, into the operation of the valve gear train. Finally, the presence of such springs is likely to be one of the primary failure modes of such a mechanism.
Accordingly, it is an object of the present invention to provide a variable valve actuation assembly which does not require a biasing spring to achieve any portion of the movement of the assembly, thereby overcoming the disadvantages of the prior art spring-type mechanisms.
It is another object of the present invention to provide a variable valve actuation assembly which is capable of being xe2x80x9cunitizedxe2x80x9d on and about the camshaft, as that term will be explained further hereinafter, which is extremely difficult to do if the mechanism is required to include a biasing spring.
Those skilled in the art have attempted to overcome the disadvantages associated with the spring-type mechanisms by developing a second category of VVA/VVT mechanisms which are classified as xe2x80x9cdesmodromicxe2x80x9d. As used herein, the term xe2x80x9cdesmodromicxe2x80x9d will be understood to mean and include a VVA/VVT type device in which the input rotation of the camshaft actuates the mechanism in both the valve opening and the valve closing directions (i.e., moving the oscillating cam in both the first direction and the second direction), thus avoiding the need to provide a return biasing spring.
Examples of such desmodromic VVA/VVT mechanisms are illustrated and described in U.S. Pat. Nos. 6,123,053 and 6,378,474. In the mechanisms of the cited patents, the mechanism is desmodromic because of the presence of a particular type of eccentric mechanism, whereby rotation of the camshaft is able to move the mechanism in both the valve opening direction and the valve closing direction, without the help of a return biasing spring. However, in the mechanisms of the cited patents, the particular eccentric mechanism selected introduces an extra output motion, generally perpendicular to the desired output motion. The mechanism must be able to effectively xe2x80x9cfilter outxe2x80x9d this extra, unproductive output motion, thus adding to the number of parts, complexity and cost of the overall mechanism.
In the prior art VVA/VVT mechanisms which are desmodromic, such as those in the cited patents, and partly as a result of the xe2x80x9cextraxe2x80x9d output motion described above, the designs typically require too many xe2x80x9cpin connectionsxe2x80x9d between adjacent members which must be free to pivot relative to each other. An excessive number of pin connections in such a mechanism adds substantially to the overall tolerance stack-up of the mechanism, which may introduce inaccuracies (looseness or xe2x80x9cslopxe2x80x9d) in the mechanism, or at the very least, may require that each such mechanism be individually adjusted after assembly. Also, such pin connections represent additional potential xe2x80x9cwearxe2x80x9d points, such that, the greater the number of pin connections in a mechanism, the greater will likely be the accumulated wear and inaccuracy over the life of the mechanism.
Accordingly, it is an object of the present invention to provide a variable valve actuation assembly of the type which is desmodromic, but which overcomes the disadvantages of the prior art devices discussed immediately above.
It is a more specific object of the present invention to provide a variable valve actuation assembly which achieves the above-stated objects, but which is relatively simple and inexpensive, and would typically not require individual adjustment at assembly.
The above and other objects of the invention are accomplished by the provision of an improved variable valve actuation assembly for use in an internal combustion engine of the type having valve means for controlling the flow to and from a combustion chamber, and a camshaft rotating in timed relationship to the events in the combustion chamber. The camshaft includes a concentric portion disposed to be concentric relative to an axis of rotation of the camshaft, and an eccentric portion disposed to be eccentric relative to the axis of rotation of the camshaft, and the eccentric portion defines an axis. The valve actuation assembly includes means defining a cam follower surface operable to provide opening and closing movement of the valve means in response to cyclic downward and upward movement of the cam follower surface. The valve actuation assembly further includes a cam member rotatably disposed about the concentric portion of the camshaft and including a cam surface disposed to be in engagement with the cam follower surface.
The improved variable valve actuation assembly is characterized by the assembly further comprising an arm assembly disposed in surrounding relationship about the eccentric portion of the camshaft. The arm assembly defines a longitudinal axis intersecting the axis defined by the eccentric portion and is perpendicular thereto. The arm assembly defines a longitudinal slot receiving the eccentric portion whereby the arm assembly is free to move transversely relative to the eccentric portion. The arm assembly defines a first relatively fixed pivot location and a second pivot location, the first and second pivot locations being longitudinally oppositely disposed about the eccentric portion. The cam member defines a connection location pivotally connected to the second pivot location of the arm assembly whereby eccentric movement of the eccentric portion about the axis of rotation of the camshaft causes the arm assembly to pivot about the first pivot location, causing oscillating rotation of the cam member.